This invention relates to "milled" tooth rotary cone rock bits and methods of manufacture therefor.
Rotary cone rock bit s for drilling oil wells and the like commonly have a steel body which is connected to the bottom of a long pipe which extends from the earth's surface down to the bottom of the well. The long pipe is commonly called a drill string. Steel cutter cones are mounted on the body for rotation and engagement with the bottom of the well being drilled to crush, gouge, and scrape rock thereby drilling the well. One important type of rock bit, referred to as a milled tooth bit, has roughly triangular teeth protruding from the surface of the cone for engaging the rock. The teeth are typically covered with a hard facing material harder than steel to increase the life of the cone. The teeth are formed into the steel cone by material-removal processes including turning, boring, and milling. Thus, the cone is referred to as a milled tooth rock bit cone because the teeth are manufactured by milling the teeth into a forged steel preform. The cones may also be referred to as steel tooth cones because they are predominantly manufactured from steel. A milled tooth rock bit cone can have 69 or more milled surfaces, five or more bores, and three or more turned surfaces. Thus, the production of a milled tooth rock bit cone is a labor intensive process, and a majority of the cost of a milled tooth rock bit cone is attributable to the labor cost. The cost is also increased by the waste of raw material which is machined away during the material removal process. The machining processes also leave sharp edges and corners on the finished cone. The sharp edges tend to crack, and the cracks propagate through the cone and through the hard facing, reducing the useful life of the cone. The sharp corners are plagued by stress concentrations which also promote cracking of the cone. Thus, teeth geometry must be limited to avoid sharp edges and corners. Further, the geometry of the teeth is limited by the capability of the milling process making infeasible some tooth shapes that increase the rate of penetration without breakage.
To address these limitations, some powder metallurgy techniques have been suggested to manufacture "milled" tooth rock bit cones. For instance, one process currently used utilizes a pattern to form a flexible mold which is filled with powdered metal. The mold is cold isostatically pressed to partially densify the powdered metal. Isostatic pressure is pressure equally applied on all sides of the mold. The partially densified part, called a green part or preform, is then heated and rapidly compressed to full density by a quasi-isostatic process.
To create the preform, the powdered metal, usually steel, is poured into the flexible mold while the mold is vibrated. Vibrating the mold during filling uniformly packs the powder in the flexible mold. The flexible mold is supported during the cold isostatic pressing by tooling which allows the deformation necessary to compress the pattern. After the mold is compressed, the preform is removed from the mold and subjected to uniform heating. Once the preform is heated, it is transferred to a central position in a cylindrical compression cavity in which it is surrounded by a bed of granular pressure transfer medium heated to approximately the same temperature as the preform. The pressure transfer medium is then axially compressed creating a quasi-isostatic pressure field acting on all surfaces of the preform. The radial pressure acting on the preform approaches a theoretical maximum of one-half of the axial pressure acting on the preform. After compression, the part is removed from the cavity and allowed to cool slowly over a two (2) hour time period. This powder metallurgy process requires two compression steps, and because the non-isostatic compression step causes a non-uniform reduction in size of the preform, its pattern is complex. The second compression process is essentially a hot pressing process, which is expensive and inefficient but only one part can be made at a time. Further, the steps required to prepare the part for the hot pressing process are complex and time consuming. Then, the process is not economical.
Other powder metallurgy process including powder injection molding have been utilized to fabricate small parts. In summary, this process begins by pelletizing or granulating a mix of powder metal and binder before injecting the pellets or granules into the mold. The mold is then removed, and the part is debinded and sintered. This process has only been utilized for small parts with thin cross-sections and heretofore has not been utilized for the production of milled tooth rock bit cones.
Thus, reduction in the required labor to fabricate a "milled" tooth rock bit cone is desirable to enhance the production rate and reduce production cost of the milled tooth rock bit cone. It is also desirable to diversify the geometric shapes of the teeth to increase the rate of penetration without the need for complexly shaped molds and preforms. Thus, the successful application of powder injection molding to produce "milled tooth rock bit cones" is desirable to bring about such an increase in the rate of penetration and decrease in the cost of rock bits which translates directly into reduction of drilling expense.